1. Field
This invention pertains to optical radiation limiters used as protective devices for imaging devices such as the FLIR (Forward Looking Infrared Imager).
2. Prior Art
Far infrared imagers must be highly sensitive in order to operate with the small temperature differentials that define many targets. Because of their sensitivity and the material used for such detector elements, e.g. mercury cadmium telluride, they are highly vulnerable to damage from low energy threat laser radiation. In an earlier patent application entitled, “Frustrated Total Internal Reflection Optical Power Limiter (FTIR)”, Ser. No. 648,140, a high energy protective device, using, e.g. nematic liquid crystals, is described, which limits the input radiation, if it exceeds a certain level, thus protecting the detector elements.
The detector material is damaged at an input fluence of between 2 and 7 J/cm2 a level easily achieved by commercially available CO2 lasers. The FTIR device has not yet been fabricated that can fully protect the detector materials in a system configuration. There are chalcogenide and vanadium oxide materials, however, that can be triggered at lower range fluence levels such that they can protect the detector material. When used alone, however, protective devices made from these materials suffer from too low a dynamic range.
An object of this invention is to extend the protection range of a device in the earlier mentioned application above, in order to close the vulnerability window, by adding a different class of light sensitive materials, similar to those mentioned above, to lower the switching threshold of the FTIR while maintaining the upper limit of the dynamic range.